Heterogeneous Catalysis DOI: 10.1002/ange.200804077 Space- and Time-Resolved Combined DRIFT and Raman Spectroscopy: Monitoring Dynamic Surface and Bulk Processes during NO x Storage Reduction Atsushi Urakawa,* Nobutaka Maeda, and Alfons Baiker Dedicated to the Catalysis Society of Japan on the occasion of the 50th anniversary Various powerful in situ spectroscopic methods and their combinations have been developed to clarify and establish relations between catalytic activity and the atomic-scale environment of catalytic active sites, particularly under actual working conditions. [1] As demonstrated recently, the addition of the space-domain into typical time-domain spectroscopy allows a deeper understanding of structural effects on catalytic activity within crystals. [2] Another relevant space domain in heterogeneous catalysis is that along the axial and radial directions in a catalyst bed of a continuous fixed-bed reactor, where prominent concentration and tem- perature gradients are known to exist. [3] For such integral-type reactors, knowledge of concentration and temperature pro- files, as well as of structural changes along the catalyst bed caused by them, is crucial for gaining insight into the governing mechanisms and for improving catalytic perfor- mance. Herein, a time-resolved study of NO x storage reduction is presented, with the addition of spatial resolution along the catalyst bed using combined diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and Raman spec- troscopy. The combined approach, employing a switch between DRIFTS and Raman spectroscopy within a single setup is known to yield rich chemical information. [4] In this study, simultaneous detections of the two spectroscopic methods within a single setup are achieved which give access to both surface and bulk information because of the greatly different local sensitivity of the two methods. Partic- ular attention is given to the position-dependent dynamic surface and bulk processes along the catalyst bed, and their relation to the overall catalytic activity. NO x storage reduction (NSR) has garnered considerable attention, owing to its NO x reduction capability in oxygen- rich atmospheres and its technical potential. Considerable effort has been undertaken in the elucidation of its underlying mechanism. [5] NSR utilizes periodic switching between fuel- lean (oxidative atmosphere) and fuel-rich (reductive atmos- phere) conditions of engines. Generally, chemical processes occurring during the two distinct periods of NSR are summarized as follows: 1) During fuel-lean periods, NO is oxidized to NO 2 over a noble metal component, such as Pt, and stored on an alkali or alkaline-earth metal component of the catalysts, such as Ba (only Ba is mentioned hereafter), in the form of nitrates. 2) During fuel-rich periods, the stored NO x is released and reduced to N 2 over the noble metal, and the Ba component is regenerated for NO x storage. Several storage and reduction mechanisms have been proposed and their relevance is still a matter of active discussion. This ambiguity is, to a large extent, caused by the difficult identification of relevant species by using solely infrared [6, 7] or Raman [8] spectroscopy. A variety of Ba species, such as nitrite, nitrate, carbonate, oxide, peroxide, and hydroxide, are involved in the processes occurring during lean–rich cycles, often appearing as overlapping signals. Herein, we demon- strate that the combination of both surface-sensitive (DRIFTS) and bulk-sensitive (Raman) time-resolved detec- tion at different catalyst-bed positions is a powerful tool to facilitate deeper understanding of complex dynamic catalytic processes and reliable band assignments. Figure 1 A shows schematically the combined DRIFTS- Raman setup configuration. The plug-flow cell design allows fast exchange of the gaseous atmosphere between lean and rich periods, and detection perpendicular to the axial direction of the catalyst bed allows gradient profiling and identification of chemical species along the bed. [9] The Pt–Ba/ CeO 2 catalyst (100 mg, 1 wt % Pt and 20 wt % Ba, 6 mm in length) was placed into the cell. Infrared light and Raman excitation laser (l = 785 nm) were focused onto the same spot of the catalyst bed through a ZnSe window. Simultaneous IR– Raman detection is possible and high-spatial resolution in the cell positioning, down to sub-micrometer levels, can be achieved. Significant spectral changes along a catalyst bed have been detected to within a less than 1 mm distance between the IR-light focal spots, in spite of the spatial averaging effects resulting from diffuse-reflectance sampling configuration. [9] Herein, we show spectra recorded at three locations of the catalyst bed for the purposes of illustration; front, middle, and back positions (0.5, 3.0, 5.5 mm from the beginning of the bed at the gas-inlet side, respectively). Figure 1 B shows NO x concentrations during NSR oper- ation (lean: NO + O 2 atmosphere, and rich: H 2 atmosphere, both balanced by He) at 623 K. During the first 30 s under [*] Dr. A. Urakawa, Dr. N. Maeda, Prof.Dr. A. Baiker Institute for Chemical and Bioengineering Department of Chemistry and Applied Biosciences, ETH Zurich Hönggerberg, HCI, 8093 Zurich (Switzerland) Fax: (+ 41) 44-632-1163 E-mail: urakawa@chem.ethz.ch Homepage: http://www.baiker.ethz.ch Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200804077. Zuschriften 9396  2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2008, 120, 9396 –9399